Conventionally, to control position of a piston in an air cylinder, position detecting device has been mounted on the side of air cylinder body such that when the piston is displaced, the position can be detected. Non-patent document 1 shows state where a contact-type sensor and a non-contact-type sensor are mounted on an air cylinder as the above detecting device.
FIGS. 8 and 9 are perspective views of the contact-type sensor, the non-contact-type sensor and the air cylinder described in the non-patent document 1. The contact-type sensor consists of magnetoresistive element.
FIG. 8 shows way how contact-type sensor 200 is mounted on air cylinder 120. In particular, approximately cylindrical contact-type sensor 200 is inserted into groove 130 formed on the side of the body of the air cylinder 120. Contact-type sensor 200 is mounted on the air cylinder 120 with setscrew 210 using screwdriver 220.
FIG. 9 shows way how the non-contact-type sensor 300 is mounted on the air cylinder 120. In particular, nut 310 is inserted into the groove 130 formed on the side of the body of the air cylinder 120. Mounting screw 330 is screwed through screw installing section 340 on the side of non-contact-type sensor 30 and spacer 320, to nut 310.
Here, a magnet, placed such that moving direction and magnetic pole direction are parallel, is generally provided on the piston 110 provided in the body of the air cylinder 120. The conventional position detecting device detects position of the piston 110 using this magnet and the sensor 200 or the sensor 300.
In addition, according to the non-patent document 2, mountable minimum stroke of the contact-type sensor 200 is approximately 10 mm and mountable minimum stroke of the non-contact-type sensor 300 is approximately 5 mm. Furthermore, hysterisis of the sensor 200 is approximately 1.5 mm and hysterisis of the non-contact-type sensor 300 is approximately 1 mm.    Non-patent document 1: SMC Corporation catalog Best Pneumatics 2 page 175    Non-patent document 2: SMC Corporation catalog Best Pneumatics 2 page 561